To maximize performance and communication capability of a wireless communication system, a multiple input multiple output (MIMO) system has drawn attention in recent years. Being evolved from the conventional technique in which a single transmit (Tx) antenna and a single receive (Rx) antenna are used, a MIMO technique uses multiple Tx antennas and multiple Rx antennas to improve transfer efficiency of data to be transmitted or received. The MIMO system is also referred to as a multiple antenna system. In the MIMO technique, instead of receiving one whole message through a single antenna path, data segments are received through a plurality of antennas and are then collected as one piece of data. As a result, a data transfer rate can be improved in a specific range, or a system range can be increased with respect to a specific data transfer rate.
The MIMO technique includes Tx diversity, spatial multiplexing, and beamforming. The Tx diversity is a technique in which the multiple Tx antennas transmit the same data so that transmission reliability increases. The spatial multiplexing is a technique in which the multiple Tx antennas simultaneously transmit different data so that data can be transmitted at a high speed without increasing a system bandwidth. The beamforming is used to add a weight to multiple antennas according to a channel condition so as to increase a signal to interference plus noise ratio (SINR) of a signal. In this case, the weight can be expressed by a weight vector or a weight matrix, which is respectively referred to as a precoding vector or a precoding matrix.
Meanwhile, an orthogonal frequency division multiplexing (OFDM) system capable of reducing inter-symbol interference with a low complexity is taken into consideration as one of post-3rd generation wireless communication systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols, and is then transmitted by being carried on N orthogonal subcarriers. The subcarriers maintain orthogonality in a frequency dimension. An orthogonal frequency division multiple access (OFDMA) is a multiple access scheme for achieving multiple access by independently providing some of available subcarriers to each user in a system using the OFDM as a modulation scheme.
One of main problems of the OFDM/OFDMA system is that a cubic metric (CM) or a peak-to-average power ratio (PAPR) can be significantly large. The CM or PAPR problem occurs when a peak amplitude of a Tx signal is significantly larger than an average amplitude, and is caused by a fact that an OFDM symbol is an overlap of N sinusoidal signals on different subcarriers. The CM or PAPR is particularly problematic in a user equipment (UE) which is sensitive to power consumption in association with battery capacity. It is important to decrease the CM or PAPR in order to decrease power consumption.
Single carrier-frequency division multiple access (SC-FDMA) is proposed to decrease the CM or PAPR. The SC-FDMA is frequency division multiple access (FDMA) combined with single carrier-frequency division equalization (SC-FDE). The SC-FDMA is similar to the OFDMA in that data is modulated and demodulated in a time domain and a frequency domain by using discrete Fourier transform (DFT). However, the SC-FDMA is advantageous to decrease Tx power since a Tx signal has a low PAPR. In particular, it can be advantageous in uplink transmission from a UE which is sensitive to Tx power in regards to the use of battery to a BS. When the UE transmits data to the BS, the transmitted data does not have a great bandwidth but it is important to provide a wide coverage capable of concentrating power. The SC-FDMA system is designed to have a small signal variation, and thus has a wider coverage than other systems when the same power amplifier is used.
Meanwhile, unlike the SC-FDMA scheme, the clustered DFT-spread-OFDM (DFT-S-OFDM) allocates (or maps) M(<N) symbol streams among N symbol streams which are DFT spread to consecutive subcarriers, and allocates (or maps) the remaining N-M symbol streams to consecutive subcarriers spaced apart by a specific interval from a subcarrier on which the M symbol streams are allocated (or mapped). Advantageously, frequency selective scheduling can be performed when using the clustered DFT-S-OFDM.
However, a single-carrier property has to be satisfied when applying the SC-FDMA scheme. The wireless communication system must be able to provide Tx diversity for decreasing the CM or PAPR by using the SC-FDMA scheme or the clustered DFT-S-OFDM scheme.
There is a need for a data transmission apparatus and method capable of providing the Tx diversity while maintaining the single-carrier property.